The prior art of implants extends back, at least, into the last century; however, only in the last twenty-five years have implants such as hip replacements, knee replacements and dental implants been widely used. These devices often employ threaded connections to fasten components of the prosthetic assembly together. Reported common problems noted by practitioners are breakage of the screw and loosening of the screw fixating the prosthesis (see U.S. Pat. No. 5,213,500, for example). It is believed that one cause of the failure of the threaded connection is the stress imposed on selected threads along the connection. A nominally manufactured component can place portions of the threads in stress conditions above the yield strength of the material, resulting in permanent deformation of the thread. This yielding may lead to a loss of pre-load tension in the connection, leading to relative motion between the joined components, and compromising the function of the prosthesis. Likewise, dynamic fatigue of the overloaded fastener can lead to catastrophic failure. These stress concentrations are compounded by the physical size restraints placed on prosthetic components. The materials which are available to the designer to choose from, to wit,--polymers, metals, and composites--oftentimes exhibit creep characteristics. The stress-raising factors encountered in implants aggravate the tendency of these materials to have time-dependent strain at stress levels below yield.
Stress placed on connectors may be radial or longitudinal and may lead to radial separation of the connector surface with attendant loss of stability in the prosthesis.